Using exogenous characteristics to identify endogenous subgroups, the approach discussed in this method note creates symmetric subsets within treatment and control groups, allowing the analysis to take advantage of an experimental design. In order to maintain treatment-control symmetry, however, prior work has posited that it is necessary to use a prediction subsample, separate from the subsample used for impact estimation in order to prevent overfitting from affecting impact estimates. 

This is the final piece in Peck's three-part series of articles that recast earlier work on analyzing ''what works'' as a call to action for evaluators and policy analysts: we can and should do better.